What is the difference between stomach and contact insecticides?

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The difference hinges on how the insect is exposed to the pesticide. Stomach (oral) insecticides must be ingested by the insect, usually when it feeds on treated bait or contaminated material, and the toxin then acts in the gut or after absorption from the gut. Contact (percutaneous) insecticides require the insect to touch the treated surface, with the chemical being absorbed through the exoskeleton as it contacts or crawls over the treated area. This pairing—ingestion for stomach poisons and contact absorption through the outer body for contact poisons—best captures the distinct routes of action. The other descriptions either mix up the routes or imply incorrect requirements (for example, stating stomach poisons must be touched or that contact poisons don’t require contact).